1. Field of the Invention
The invention relates to telecommunications networks and in particular to such networks incorporating a private branch exchange (PBX).
2. Related Art
The deployment of telephone services typically requires the installation of costly copper lines from a central office (CO) to, for example, a business facility with a private branch exchange (PBX). Today, the wireless link offers a reasonable alternative to copper analog lines for single wireline phones.
Businesses often employ a private branch exchange, which is typically a manual or dial exchange connected to the public telephone network and is located on the premises of the business. A PBX, which is a switching center, typically has a trunk connection to a central office (CO), which is also a switching center. Conventional systems cannot use a wireless link between the PBX and the CO because it would be necessary for the wireless links to appear to the PBX as central office trunks with direct inward dialing (DID) capability, since wireless links do not support incoming call destination addresses.
A central office can communicate with a PBX on a direct inward dial (DID) trunk, a direct outward dial (DOD) trunk, a digital T1 trunk, or a Primary Rate Interface (PRI), which is an Integrated Services Digital Network (ISDN) trunk. Signalling between the CO and the PBX is typically accomplished by multiple frequency (MF), dual tone multiple frequency (DTMF) or dial pulse methods when using analog trunks. In digital systems, signalling is accomplished over a digital signalling channel. A trunk interface, which drives a PBX, is distinguished from a loop interface, which drives telephone sets. In a trunk interface, the CO provides a destination address and the PBX provides the ringing function. Therefore, the trunk interface can be used by many telephone sets at many destination addresses. In a typical loop circuit, all the telephone sets have the same destination address. A radio unit which provides a loop interface to a telephone connected in an RJ-11 interface has provided individual communication between a public switched telephone network (PSTN) and such telephones. Such an arrangement is not suitable for a PBX because it responds to only one phone number, i.e., there is one radio link required per telephone.
Thus, a typical PBX communicates with a central office over copper wire. Installation of such hard-wired facilities is expensive and time consuming. Especially in developing countries, such hard sired facilities take a long time to install and are subject to outage induced by natural and man-made disruptions.
In addition, the emergence of new, low power, wireless technologies has facilitated the development of in-building wireless telephone services, such as CT2, CT2Plus and DECT. Within such in-building wireless service zones, these technologies provide incoming and outgoing call service. However, a subscriber has no coverage outside such zone from these low power sets and must rely on different wireless technologies, such as cellular or GSM, outside the zones. This has the disadvantage of requiring the user to use a separate phone with a different number outside the in-building zone.